1. Field of the Invention
Embodiments of the present invention generally relate to microelectromechanical systems (MEMS) and, more specifically, to resonator electrode shields that are configured to reduce interfering signals.
2. Description of the Related Art
Microelectromechanical system (MEMS) devices are currently being developed for a wide variety of applications. One such device is a MEMS resonator, which can be used in the timing circuitry of electronic devices to generate a timing signal. MEMS resonator systems typically include one or more electrodes that drive the motion of a MEMS resonator (referred to herein as “drive electrodes”). As is well-known, when a voltage is applied to a drive electrode, a charge accumulates on the electrode that applies an electrostatic force between the electrode and an opposite charge built up on the MEMS resonator structure. By applying a time-varying voltage signals to the drive electrode, often in combination with a DC voltage, a time-varying electrostatic force can be generated that capacitively couples mechanical energy to the active elements of the MEMS resonator structure, causing the active elements to resonate (the term “active elements” is defined herein to be the one or more elements of a the MEMS resonator structure whose motion causes the MEMS resonator to generate the desired timing signal). In addition one or more sense electrodes generate or conduct a time-varying current as a result of capacitive coupling between the moving active elements of the MEMS resonator structure and the sense electrodes. Ultimately, the timing signal is extracted from the time-varying current conducted by the sense electrodes. The sense and drive electrodes do not need to be paired. For example, the number of sense and drive electrodes may differ, or the electrodes may be used for both sense and drive. Alternatively, the active elements can be used as the drive or sense electrodes.
The desired capacitive coupling from the drive electrodes to the active elements of the MEMS resonator structure to the sense electrodes may be accompanied by additional coupling from drive to sense electrodes directly or through other elements of the MEMS resonator system. These signal paths may create interfering signals that can compromise the overall integrity of the generated timing signal.
One kind of an interfering signal, referred to herein as an “induced current,” can arise when time-varying voltage signals applied to the drive electrodes causes time-varying currents to be induced within one or more non-active elements of the MEMS resonator, support, or other structures (i.e., elements not responsible for generating the desired timing signal such as the elements that provide structural support for the MEMS resonator system). These induced currents can alter the voltage on the MEMS resonator structure, which can capacitively couple to the sense electrodes. For example, when current is induced in support structures, these currents can cause voltages to be impressed on the resonator structures. The changing voltage on the resonator relative to the sense electrodes can induce a sense current.
A second kind of an interfering signal, referred to herein as a “feed-through current,” can arise when the one or more drive electrodes couple capacitively directly to the one or more sense electrodes. In such a case a capacitively coupled signal can traverse past the resonator structure from drive to sense independently of the mechanical motion of the resonator.
Yet another kind of a interfering signal, referred to herein as a “spurious resonance current,” can arise due to the mechanical motion of non-active elements of the MEMS resonator structure. Such non-active elements may include support or auxiliary elements within the overall MEMS resonator system. An electrostatic force from one or more drive electrodes can cause one or more non-active elements, such as isolating springs, to move or resonate. The mechanical motion may be transduced by a nearby sense electrode, causing the sense electrode to conduct additional current, separate from the desired current produced in response to the desired motion of the active elements of the MEMS resonator structure.
There is additionally a form of spurious resonance current that can be caused from an undesired forcing of a resonant structure that excites an undesired mechanical mode. This differs from the previously described spurious resonance in that it can be in the primary resonance structure, rather than for instance a support or an isolation spring. To avoid this, the resonant structure in some cases must have a carefully controlled forcing and sensing geometry to assure that it only is exited in a desired mode.
These spurious currents can decrease the signal quality of the oscillator that is built with the MEMS resonator. This can occur in various ways that are particular to the type or cause of the currents. The descriptions below are examples and are not to be understood as exhaustive cases.
One consequence of the interfering currents can be reduced signal to noise ratio (SNR) of an oscillator built with the MEMS resonator. The interfering currents can increase the electrical noise in the MEMS resonator system since they are not from the intended motion of the active elements of the MEMS resonator structure.
Another consequence of interfering currents is decreased stability of the output frequency of an oscillator built with the MEMS resonator. The interfering currents can shift the phase of the MEMS resonator or sustaining circuit and shift the frequency of the oscillator. For example, feed-through current is normally 90° out of phase with the MEMS on-peak resonance current. When these currents are summed the aggregate current is not in phase at the MEMS resonance peak. The oscillation criteria can then only be satisfied if the circuitry's or the resonator's phase is shifted. This shift can change with temperature or time and thereby affect the frequency stability.
Another consequence of interfering currents is that they may be large enough to cause the overall oscillator to lock onto an undesirable resonant mode at an undesired frequency. For example, spurious currents can excite an undesired resonance in a support or isolation structure that can be mistaken for the desired resonance.
One way to minimize the interfering signals described herein is through differential cancellation, where differential drive electrodes are configured to apply opposite charges on the MEMS resonator structure, and/or differential sense electrodes are configured to reject common-mode coupled currents. However, not all electrodes and not all mechanical arrangements can be configured for differential signaling. In addition, there may be situations when one polarity of the differential drive and/or sense electrodes couple more strongly to the resonator structure than the other polarity. This can occur for instance when the electrodes are not fully symmetrical. In such situation differential cancellation may not be effective or may not be as effective as desired.
Another way to minimize interfering signals, particularly in the case of induced current is to decrease the induced voltage on the MEMS resonator by decreasing the structural impedance between the MEMS resonator and either the bias or ground. Such an approach may be implemented with carefully designed electrical contacts, by increasing the doping within the MEMS resonator structure, and/or modifying the mechanical design of the MEMS resonator system. However, limitations on electrode design, the mechanical and fabrication issues associated with achieving high doping levels within the MEMS resonator structure, and/or the mechanical constraints on the design place a limit on how much the overall impedance may be decreased.
As the foregoing illustrates, what is needed in the art is a different way to decrease the adverse effects of interfering signals in a MEMS resonator system.